Field of the Invention
The present invention relates to a toner and a method for producing the toner.
Description of the Related Art
The image forming method for, for example, electrophotography, electrostatic recording and electrostatic printing includes a development step in which a toner contained in a developer is attached once to an image bearing member such as photoconductor on which a static charge image is formed, a transferring step in which the toner is transferred from the photoconductor to a transfer medium such as transfer paper, and a fixing step in which the toner is fixed on the paper.
Known developers are classified into the following two types: a two-component developer containing a magnetic carrier and a toner, and a one-component developer without the magnetic carrier (a magnetic toner or a non-magnetic toner).
In recent years, there has been increasingly a demand for higher quality images, and in particular, to realize forming a high-precision color image, small sizing and spheroidizing of the toner are under way. The small sizing enables excellent dot-reproductivity, and the spheroidizing makes it possible to improve developing properties and transferring properties.
In order to provide such a small-sized toner, in recent years, many methods for producing a toner in which toner particles are granulated in liquid have been proposed such as polymerization methods (e.g., a suspension polymerization method, an emulsion polymerization method, and a dispersion polymerization method). In the polymerization methods, upon forming the toner particles, the toner particles are polymerized in a medium to which a surfactant and a dispersion stabilizing agent have been added in order to control the particle diameter or the shape of the toner particles.
However, in the case where the surfactant and the dispersion stabilizing agent remain in the thus produced toner, the toner is adversely affected in chargeability, i.e., the toner is lowered in charge and resistance. Especially, the toner is greatly adversely affected under a high temperature and high humidity condition. Examples of known dispersion stabilizing agents include a water-soluble polymeric material which forms a protective colloid phase to thereby mainly prevent particles from coalescing with each other and from precipitating; and a poorly water-soluble inorganic compound which mainly generates electrostatic repulsion for dispersion stabilization.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2007-127678 describes a use of a poorly water-soluble inorganic compound colloid because a water-soluble polymeric material has the following disadvantages. Firstly, the water-soluble polymeric material adhered onto the surfaces of polymeric particles is difficult to be removed by a work-up such as a post-polymerization washing. In addition, upon polymerizing the polymeric particles, fine particles having particle diameters of sub-micron level are by-produced besides intended polymeric particles. Therefore, upon filtering off the polymeric particles from an aqueous medium, a filter medium is clogged with some free fine particles, which lowers a filtration rate and thus production efficiency.
JP-A No. 07-152202 discloses a method for obtaining toner particles as follows. An oil phase (organic solvent liquid) is obtained by dissolving or dispersing a toner material containing a polyester resin and a colorant in a solvent in which the polyester resin can be dissolved. Then, the oil phase is dispersed in an aqueous medium containing an inorganic dispersing agent, followed by granulating. Finally, the inorganic dispersing agent is washed off with hydrochloric acid to thereby obtain toner particles.
JP-A No. 07-319205 discloses a method for obtaining toner particles as follows. At first, a resin suspension liquid is obtained by adding a colored resin solution containing a toner material to an aqueous dispersion liquid containing an inorganic dispersing agent. Then, the resin suspension liquid is desolvated. Concentrated sulfuric acid is added thereto to thereby dissolve the inorganic dispersing agent. The resultant toner is washed by adding to deionized water and stirring until conductivity thereof becomes 1 μmS/cm to 100 μmS/cm. However, in the above method, only dispersion stabilizing agents remaining in the close proximity to the surfaces of the toner particles are removed. That is, the dispersion stabilizing agents remaining inside the toner particles are removed incompletely, which is insufficient to maintain chargeability over a long period of time.
JP-A No. 10-319624 proposes a method for obtaining toner particles as follows. Toner particles are washed with water, and then redispersed into washing water. Then the toner particles are heated to a temperature equal to or higher than the glass transition temperature of a resin contained in toner particles, stirred, and washed. Thus, a solution of the toner particles having electroconductivity of 100 μs or less and surface tension of 20 mN or more is obtained.
The above proposed technique focused on that a change of toner chargeability under a high temperature and high humidity environment depends on a trace amount of ion remaining on the surfaces of the toner particles which is unavoidably occurred in the manufacturing process. Based on that, deionized water was used for washing off the dispersion stabilizing agent and the surfactant after granulation, and water-washing was performed until the amount of ion components such as a halogen ion and a sodium ion remaining on the surfaces of the toner particles is decreased to fall within a predetermined range. It was argued that the above technique allowed the obtained toner to maintain excellent chargeability even after a long-term storage under a high temperature and high humidity environment. However, the technique is still insufficient in that the dispersion stabilizing agents remaining inside the toner particles cannot be removed. Additionally, when once-formed toner base particles are heated in liquid to a temperature equal to or higher than the glass transition temperature, undesired results are generated such as a deformation of the particles or a change of the particle diameter.
JP-A No. 2006-227592 discloses that a washing end point cannot be determined unless ion-exchanged water is increased in temperature to the temperature equal to or higher than the glass transition temperature of a binder resin. As such, the surfactant and the dispersion stabilizing agent are very difficult to be removed.
On the other hand, JP-A No. 2005-173578 discloses a use of a water-soluble polymeric material as the dispersion stabilizing agent. However, the water-soluble polymeric material is not used at a granulation step prior to a desolvation, but used at a deformation step after producing toner particles.